This is a renewal application to study the underlying mechanisms of activation-induced cell death (AICD) in T cells, one of the primary mechanisms of clonal deletion and therefore central to our understanding of the immune system. Here, we define AICD as cell death proceeding as a consequence of cell (in this case, T cell) activation. Our proposed studies are concerned here with the role of AICD, per se, in the homeostatic process of apoptosis induced by T cell activation under conditions limiting engagement of nuclear factor-kB (NF-kB). Our studies are based on the central hypothesis that the function of NF-kB during T cell activation serves to preserve survival and proliferation by inhibition of the p53-family protein p73. Our goals are to elucidate a pathway leading from T cell activation to neutralization of p73 and its downstream effectors, and examine the consequences of disruption of this pathway on T cell survival, proliferation, and memory. The role of this pathway in normal conditions of immunization and tolerance induction will be explored. In particular, we will ask: 1. What is the role of the NF-kB-MDM2 axis in survival of activated T cells? Based on our preliminary observations and those of others, we postulate that upon T cell activation, MDM2 is induced by NF-kB and functions to neutralize the pro-apoptotic effects of p73, also engaged by T cell activation. In this aim we will probe the pathway from engagement of CD3-TCR and CD28 to NF-kB and how the latter targets MDM2, as well as the consequences of failures in this pathway on T cell survival. We will further test the idea that this mechanism functions to eliminate T cells that have been only partially activated for a variety of reasons. This will be put into the context of T cell survival during immune responses. 2. How does the MDM2-p73 interaction contribute to survival of activated T cells? Previous studies and our preliminary data strongly suggest that ACID in T cells defective for NF-kB activation is mediated by p73. Our observations indicate that NF-kB-induced MDM2 acts to limit this p73-mediated cell death. In this aim we will explore the regulation of p73 by MDM2 in T cell survival, proliferation, and function in the contact of NF-kB activation, using pharmacologic and genetic approaches to disrupt this interaction. 3. How does deregulated p73 cause AICD? The p53 family proteins function primarily as transcription factors, and we propose that the effect of deregulated p73 during T cell activation is due to such activity. Here, we will focus on one potential p73 target, identified by a candidate approach, Bim. We will explore the role of Bim in activation-induced, p73-dependent apoptosis in activated T cells, in relation to engagement of NF-kB. In addition, the possibility that p73 also engages cell cycle inhibition/senescence will be explored in the context of T cell anergy in the absence of apoptosis. The proposed research integrates our understanding of T cell function with our current ideas of the control of apoptosis. Although admittedly ambitious, our program of research will allow us to dissect the central mechanisms that contribute to the important biological phenomenon of AICD in T cells. Our laboratory has been intimately involved in the exploration of the phenomenon of AICD since its discovery, and the current proposal is our only application (funded or submitted) that seeks to explore this general phenomenon in detail. By focusing our efforts on the NF-kB-MDM2-p73-Bim pathway, we will make fundamental progress in elucidating one mechanism of AICD and how it influences immune responses. PUBLIC HEALTH RELEVANCE: This is a renewal application of a project to study activation-induced cell death in T cells, focusing specifically on a novel pathway in which the transcription factor, Nuclear Factor-kappaB, induces expression of MDM2, which in turn inhibits the function of the p53 family member, p73. When this pathway is disrupted for any reason, p73 induces expression of the pro-apoptotic protein, Bim, thereby killing the cell. This novel pathway has fundamental implications for our understanding of T lymphocyte responses under different conditions, and will have impact on our ability to manipulate the immune response.